Contrast agents are known, for example, from DE 44 33 564 A1, WO 00/16811 or DE 100 02 939 C1.
The result of radiographic methods such as, for example, computed tomography, mammography, angiography, X-ray inspection technology or comparable methods, is firstly the representation of the attenuation of an X-ray along its path from the X-ray source to the X-ray detector (projection image). This attenuation is caused by the transradiated media or materials along the beam path, and so the attenuation can also be understood as a line integral over the attenuation coefficient of all pixels along the beam path. Particularly in the case of tomography methods, for example in X-ray computed tomography, so-called construction methods can be used to calculate backward from the projected attenuation data to the attenuation coefficients (μ) of the individual pixels, and thus to achieve a substantially more sensitive examination than from purely evaluating the projection images.
Instead of the attenuation coefficient, use is generally made for the purpose of representing the attenuation distribution of a value normalized to the attenuation coefficient of water, the so-called CT number. This is calculated from an attenuation coefficient μ, currently determined by measurement, and the reference attenuation coefficient μH2O using the following equation:
                              C          =                      1000            ×                                                            μ                  ⁢                                                                          ⁢                                      μ                                                                  H                        2                                            ⁢                      O                                                                                        μ                                                            H                      2                                        ⁢                    O                                                              ⁡                              [                HU                ]                                                    ,                            (        1        )            where the CT number C is in Hounsfield [HU]. A value of CH2O=0 HU is used for water, and for air a value CL=−1000 HU.
Since both representations can be transformed into, or are equivalent to, one another, the generally selected term of attenuation value or attenuation coefficient designates below both the attenuation coefficient μ and the CT value. Furthermore, the terms material and tissue are used exchangeably in the context of the subject of this description of embodiments of the invention. It is assumed that in the context of a medically indicated examination a material can be an anatomical tissue, or conversely for material and safety testing a tissue is understood to be any desired material of an examination object.
Although the informativeness of an image based on the local attenuation coefficient (μ) is clearly enhanced, problems can nevertheless arise with interpreting an image in the individual case. Specifically, a locally increased attenuation value can be ascribed either to materials of higher atomic number such as, for example, calcium in the skeleton or iodine in a contrast agent, or to an increased soft part density such as, for example, in the case of a pulmonary nodule. The local attenuation coefficient μ at the location {right arrow over (r)} is dependent on the X-ray energy E irradiated into the tissue and/or material and the local tissue and/or material density ρ in accordance with the following equation:
                              μ          =                                    μ              ⁡                              (                                  E                  ,                                      r                    ->                                                  )                                      =                                          μ                ρ                            ⁢                              (                                  E                  ,                  Z                                )                            ×                              ρ                ⁡                                  (                                      r                    ->                                    )                                                                    ,                            (        2        )            with the energy-dependent and material-dependent mass attenuation coefficient
      μ    ρ    ⁢      (          E      ,      Z        )  and the (effective) atomic number Z.
The energy-dependent X-ray absorption of a material as determined by its effective atomic number Z is therefore superimposed on the X-ray absorption influenced by the material density ρ. Materials and/or tissue of different chemical and physical composition can therefore exhibit identical attenuation values in the X-ray image. Conversely, by contrast, it is not possible to deduce the material composition of an examination object from the attenuation value of an X-ray picture.
Methods for representing characteristic values of materials are required in order to solve this problem. In conjunction with computer-aided tomography methods, it is known, for example from U.S. Pat. No. 4,247,774, to use mutually different X-ray spectra or X-quantum energies to produce an image.
Such methods are generally denoted as dual-spectrum CT. They utilize the energy dependence, governed by atomic number, of the attenuation coefficient μ, that is to say they are based on the effect that materials and tissue of higher atomic number absorb low-energy X-radiation substantially more intensely than do materials and/or tissues of lower atomic number.
By contrast, in the case of higher X-ray energies the attenuation values are equal and are largely a function of material density. In the case of dual-spectrum CT, the differences in the images recorded for different X-ray tube voltages are then calculated, for example.
Unless otherwise specified, in the context of this description of embodiments, the term atomic number is used not in the strict sense as referring to elements. Instead it denotes an effective atomic number of a tissue, or material, that is calculated from the chemical atomic numbers and atomic weights of the elements participating in the structure of the tissue and/or material.
Even more specific statements are arrived at when, in addition, the method of so-called base material decomposition is applied in the case of X-ray pictures, for example as described in W. Kalender et al. in “Materialselektive Bildgebung und Dichtemessung mit der Zwei-Spektren-Methode, I. Grundlagen und Methodik” [“Material-selective imaging and density measurement with the dual-spectrum method, I. fundamentals and methodology”], W. Kalender, W. Bautz, D. Felsenberg, C. Süβ and E. Klotz, Digit. Bilddiagn. 7, 1987, 66-77, Georg Thieme Verlag.
In this method, the X-ray attenuation values of an examination object are measured with the aid of X-ray beams of lower and higher energy, and the values obtained are compared with the corresponding reference values of two base materials such as, for example, calcium (for skeletal material) and water (for soft part tissues). It is assumed that each measured value can be represented as a linear superposition of the measured values of the two base materials. For example, a skeletal component and a soft tissue component can be calculated for each element of the pictorial representation of the examination object from the comparison with the values of the base materials, the result being a transformation of the original pictures into representations of the two base materials of skeletal material and soft part tissue.
The base material decomposition and the dual-spectrum method are therefore suitable for telling apart or distinguishing predefined anatomical structures or types of material in human and animal tissues having a sharply different atomic number.
German Patent Application with the application number 101 43 131 discloses a method whose sensitivity and informativeness further exceeds the base material decomposition and, for example, enables a functional CT imaging of high informativeness. It permits the calculation of the spatial distribution of the mean density ρ({right arrow over (r)}) and of the effective atomic number Z({right arrow over (r)}) from an evaluation of the spectrally influenced measured data of an X-ray apparatus. Very good contrasts are yielded thereby, for example with reference to the chemical and physical composition of the examination object. For example, the representation of the distribution of the atomic number in the tissue permits, inter alia, insight into the biochemical composition of an object being examined, contrasts based on chemical composition in organs previously represented as of homogeneous density, a quantitative determination of body constituents such as, for example, iodine or the like, and removal of instances of calcification by segmentation on the basis of the atomic number.